The present invention concerns iron based powder compositions for the preparation of compacted and sintered products having improved properties. More specifically the invention concerns the influence of the largest particles of the lubricant and the iron based powder used in the composition on the dynamic properties of the final products.
Fatigue performance of sintered steels are influenced by several factors which are interacting. The density was early established as one of the most influential factors together with the microstructure and alloy element content but also homogeneity, pore size and pore shape are known to influence the dynamic properties. This makes fatigue performance one of the most complex properties of PM materials.
An object of the present invention is to improve the dynamic properties of sintered steels, specifically sintered steels having a density between 6.8 and 7.6 g/cm3.
Another object of the invention is to eliminate the influence of the particle size of the lubricant on the dynamic properties, especially the fatigue strength of the sintered parts.
A third object is to provide a method of improving the fatigue strength by selecting the particle size of the lubricant in view of the particle size of the iron powder.
According to the invention it has now been found that, even if the amount of the very largest particles of a lubricant constitutes a negligible or almost negligible fraction of the lubricant particle size distribution as well as of the amount of the lubricant, this fraction has an unexpectedly large detrimental effect on the pore size and accordingly on the dynamic properties.
Similarly it has been found that the very largest particles of the iron powder, i.e. the maximum size of the iron powder has an unexpectedly large detrimental effect on the dynamic properties. Thus in order to get improved dynamic properties the maximum size of the lubricant particles as well as the maximum size of the iron powder should be reduced. For presently commercially used ferrous based press powders this means that the maximum particle size of the lubricant should be less than about 60 xcexcm as measured by laser diffraction measurement.
In order to achieve the best dynamic properties for a given iron based powder (at a given density) a relationship between the maximum size of the particles of the lubricant and the maximum size of the particles of the iron based powder has also been established. The term xe2x80x9cmaximum sizexe2x80x9d as used in this context is defined in the formula below.
In accordance with the invention it has been found that the particle size of the lubricant in a composition including the lubricant and an iron based powder for powder metallurgical preparation of compacted and sintered products should be selected so that the largest pores of the compacted and sintered product prepared from this composition should be equal to or less than the largest pores obtained in a compacted and sintered product prepared from the same composition without the lubricant, which in practice means that the compaction is performed in a lubricated die.
Empirically we have found the following relationship between the largest lubricant particles and the largest iron powder particles in order to avoid the influence of the lubricant on the size of the largest pores.
Lubmaxxe2x89xa60.31xc3x97Femaxxe2x88x9226, wherein 
Lubmax is the lubricant particle size in xcexcm whereas 99.99% of the lubricant is finer.
Femax is the iron particle size in xcexcm whereas 99.99% of the iron powder is finer
(this could also be expressed as Lubmax is the size of the largest one hundredth of a percent fraction of lubricant particles in xcexcm,
Femax is the size of the largest one hundredth of a percent fraction of the particles of the iron based composition in xcexcm). This means that the maximum particle size of the lubricant as defined above should be less than about 0.3 of the maximum size of the iron or iron-based particles.
The iron based powder according to the invention may be an alloyed iron based powder, such as a prealloyed iron powder or an iron powder having the alloying elements diffusion-bonded to the iron particles. The iron based powder may also be a mixture of an essentially pure iron powder and the alloying elements.
The alloying elements which can be used in the compositions according to the present invention may be one or more elements selected from the group consisting of Ni, Cu, Cr, Mo, Mn, P, Si, V and W. The particle sizes including the maximum particle sizes of the alloying elements are smaller than those of the iron or iron-based powder. The various amounts of the different alloying elements are between 0 and 10, preferably between 1 and 6% by weight of Ni, between 0 and 8, preferably between 1 and 5% by weight of Cu, between 0 and 25, preferably between 0 and 12% by weight of Cr, between 0 and 5, preferably between 0 and 4% by weight of Mo, between 0 and 1, preferably between 0 and 0.6% by weight of P, between 0 and 5, preferably between 0 and 2% by weight of Si, between 0 and 3, preferably between 0 and 1% by weight of V and between 0 and 10, preferably between 0 and 4% by weight of W.
The iron based powder may be an atomised powder, such as a wateratomised powder, or a sponge iron powder.
The particle size of the iron based powder is selected depending on the final use of the sintered product and, according to the present invention is has been found that also the maximum particle size of the iron based powder has an unexpectedly large detrimental effect on the dynamic properties of the sintered product.
The type of lubricant is not critical and the lubricant may be selected from a wide variety of solid lubricants. Specific examples of suitable lubricants are conventionally used lubricant such as Kenolube(copyright), Metalub, (both available from Hxc3x6ganxc3xa4s AB Sweden) H-Wachs(copyright) (available from Clariant), and zinc stearate (available from Megret). The amount of the lubricant may vary between 0.1 and 2, preferably between 0.2 and 1.2. Furthermore the vaporising temperature of the lubricant should be below the sintering temperature of the compacted part. Presently used lubricant which may be used according to the present invention have vaporising temperatures less than about 800xc2x0 C.
The amount of graphite varies between 0 and 1.5, preferably between 0.2 and 1% by weight of the composition. Also, the maximum particle size of the graphite powder should be equal to or smaller than the maximum particle size of the lubricant.
In addition to the iron based powder, optional alloying elements, graphite and lubricant(s) the compositions according to the invention may also include optional conventionally additives, such as MnS, Mnx(trademark).
The improved dynamic properties which can be obtained according to the present invention are especially interesting in sintered products having densities between 6.8 and 7.6 g/cm3, especially between 7.0 and 7.4/cm3.
Examples of preferred iron based powders plus preferred amounts of graphite follows below:
Iron+4% Ni+1.5% Cu+0.5% Mo where the alloying elements are diffusion bonded to the iron particle mixed with 0.4 to 1% graphite.
Iron+1.75% Ni+1.5% Cu+0.5% Mo where the alloying elements are diffusion bonded to the iron particle mixed with 0.4 to 1% graphite.
Iron+5% Ni+2% Cu+1% Mo where the alloying elements are diffusion bonded to the iron particle mixed with 0.4 to 1% graphite.
Iron prealloyed with 1.5% Mo and 2% Ni diffusion bonded to the iron/Mo particle which are mixed with 0.4 to 1% graphite.
Iron prealloyed with 1.5% Mo and 2% Cu diffusion bonded to the iron/Mo particle which are mixed with 0.4 to 1% graphite.
Iron prealloyed with 1.5% Mo and 2% Cu and 4% Ni diffusion bonded to the iron/Mo particle which are mixed with 0.4 to 1% graphite.
Iron prealloyed with 1.5 or 0.85% Mo and mixed with 0.4 to 1% graphite.
Iron prealloyed with 3% Cr and 0.5% Mo and mixed with 0.2 to 0.7% graphite.
These iron based powders all contains powders with a particle size below 212 xcexcm sieved.
According to one especially preferred embodiment of the invention the maximum particle size of the iron based powder should be less than about 220 xcexcm (which is obtained for e.g. Astaloy Mo xe2x88x92106 xcexcm, through sieve analysis), and for this powder the maximum particle size of the lubricant should be less than 60 xcexcm as measured by laser diffraction measurement.
The compacting and sintering steps for the preparation of the final products, which are distinguished by essentially the same or better dynamic properties as obtained for the same composition but without lubricant are performed under conventional conditions, i.e. the compaction is carried out at pressures between 400 and 1200 MPa and the sintering is performed at temperatures between 1100 and 1350xc2x0 C.
The invention is further illustrated by the following non limiting examples.